1. Field of the Invention
The present invention relates to a PWM controller and a power converter, especially to a PWM controller detecting temperature and AC line via a single pin, and a power converter including the PWM controller.
2. Description of the Related Art
To prevent damage caused by high temperatures, general power converters are implemented with an over-temperature protection function. Please refer to FIG. 1, which illustrates a circuit diagram of a prior art AC-to-DC converter having an over-temperature protection function. As illustrated in FIG. 1, the prior art AC-to-DC converter includes a PWM controller 100, an NMOS transistor 110, and a negative-temperature-coefficient resistor 120.
The PWM controller 100 has an output pin (number 5) for providing a PWM signal to drive the NMOS transistor 110, and has a pin (number 1) for detecting temperature.
The negative-temperature-coefficient resistor 120, which is preferably located beside the NMOS transistor 110, has a resistance dependent on temperature—the higher the temperature, the smaller the resistance. When temperature exceeds a threshold temperature, a reduced resistance smaller than a threshold resistance will be detected by the PWM controller 100, and the PWM controller 100 will shut down the PWM signal accordingly.
In addition to over-temperature protection, AC line brownout protection capable of shutting down power transmission when AC line voltage drops to under a threshold is also important for AC-to-DC converters. Please refer to FIG. 2, which illustrates a circuit diagram of a prior art AC-to-DC converter having an AC line brownout protection function. As illustrated in FIG. 2, the prior art AC-to-DC converter includes a PWM controller 200, an NMOS transistor 210, a bridge rectifier 220, a diode 230, a capacitor 240, and resistors 251-252.
The PWM controller 200 has an output pin (OUT) for providing a PWM signal to drive the NMOS transistor 210, and has a pin (BNO) for receiving a sensed signal corresponding to a voltage amplitude of an AC power.
The bridge rectifier 220 has diodes 221-224 for performing a full-wave rectification of the AC power.
The diode 230 and the capacitor 240 are used to generate a first voltage by rectifying and filtering a line voltage of the AC power.
The resistors 251-252 are used to divide the first voltage to generate the sensed signal.
When the voltage amplitude of the AC power decreases to cause the sensed signal to fall below a threshold, the PWM controller 200 will shut down the PWM signal to prevent the NMOS transistor 210 from getting damage (As is known, a smaller amplitude of the AC power will result in a larger power consumption on the NMOS transistor 210, so the PWM signal has to be shut down when the sensed signal falls below the threshold to prevent the power consumption of the NMOS transistor 210 from exceeding a rated value).
Due to a limited pin count of the package for PWM controllers, general PWM controllers are either designed to have an over-temperature protection function as illustrated in FIG. 1, or designed to have an AC line brownout protection function as illustrated in FIG. 2. To integrate these two functions in one chip, a larger pin count and therefore a larger package has to be used. However, this will increase package cost and board area.
To solve the foregoing problems, a novel PWM controller and a corresponding power conversion architecture are needed.